There is a significant need for controlling high voltage direct current with a physically small switching device, such as a relay. The problem involved in satisfying this need, however, is that as the contacts of a relay are opened or closed, the electrical discharge created by the interruption of the electrical current due to contact bounce or the opening of the contacts causes heating which burns and erodes the electrodes, leading to welding and destruction of the relay contacts. A number of attempts have been made in the prior art to solve this or similar problems. For example, U.S. Pat. No. 4,250,531 to Ahrens discloses a switch-arc preventing circuit which employs a varistor in shunt connection across the power electrodes of the switching transistor to limit inductive spikes. A defect of this approach is that the relay is not actually controlling the power but is instead providing a control signal to power switching transistors. Power switching transistors cannot handle the high power switching requirements which currently exist. Another approach attempting to solve the arc suppression problem is shown in U.S. Pat. No. 3,912,941 to Passarella, which discloses an isolation circuit for arc reduction in a DC circuit. This circuit employs a transistor in which the collector and emitter are connected in series with the power supply and the load while the base is connected through a resistive gating circuit to the switch. Once again, the transistor switch switching contacts isolated from the load are not arc suppressed. And furthermore, the load current is limited by the transistor switch. Still a further attempt to solve the arcing problem is described in U.S. Pat. No. 3,184,619 to Zydney, which discloses a contact noise suppressor. When the contacts open, the negative potential provided by the source is disconnected from the load circuit. Contact bounce, however, is not arc suppression and the patented device serves only to reduce load sensitivity to erratic closure or bounce of the contacts and does not serve to suppress the arcing associated with switching large direct current power. The disclosed circuit is basically a pulse stretcher which is configured for normally closed contacts and does not effectively suppress arcs. Furthermore, the timing for the circuit is controlled by a resistor and is relatively slow and cannot provide for a rapid recovery to defeat contact bounce effects. Another attempt of solving the problem of arc suppression has been described in U.S. Pat. No. 3,075,124 to Bagno, which discloses a contact protection circuit which is connected in series between the power supply and the protected contacts. The protective circuit must pass all power through the active device and therefore arc suppression upon opening of the contacts would be almost nonexistent. This is because charges are stored in the active devices and thus they cannot reduce the energy at the contacts unless there is a very low power level.
U.S. Pat. No. 3,504,233 discloses a pair of oppositely connected SCRs 20 and 21, for shunting an AC circuit breaker 10. The gates of the SCRs are directly connected to the contact members 18 of the circuit breaker 10. During arcing of the contact member 13, the SCRs will conduct (depending upon the specific half cycle of the AC current). This circuit has the limitation that it must have alternating current in order to operate, since the only way to make the SCRs 20 and 21 turn off after they have shunted the current around the circuit breaker 10, is for the power supply to go through the zero cross-over which only occurs in AC power supplies, not DC power supplies. The circuit disclosed in the patent would not work for DC power supplies. Another patent making a similar disclosure is U.S. Pat. No. 3,639,808, which also suffers from the same limitation.
U.S. Pat. No. 3,555,353 discloses a TRIAC D having its gate electrode 22 directly connected to a relay coil RW controlling the AC relay switch RK, such that the TRIAC is turned on to suppress arcs when the switch is closed. There is no protection for the switch on the opening thereof because the TRIAC is off first before the contacts open. Still further, the circuit would not work for DC power supplies, since once again, the TRIAC would not turn off without a zero transition for the power supply which is only available for AC power supplies.
U.S. Pat. No. 3,474,293 discloses another TRIAC arc suppressing circuit which protects only on the opening of the switch but does not protect on the closing of the switch. Once again, the circuit would not work in a DC environment since the TRIAC must be turned off when the AC power supply goes to zero, a situation which is not present in a DC system.
In summary, the prior art has been unable to provide an adequate solution to the problem of active arc suppression for switching DC current circuits.